This invention relates to an electromagnetic drive actuator.
FIG. 1 is a perspective view showing a conventional electromagnetic drive actuator as disclosed in Japanese Utility Model Application (OPI) No 63881/1983 (the term "OPI" as used herein means an "unexamined published application"), and FIG. 2 is an explanatory diagram for the operation thereof.
In these figures, reference numeral 1 designates permanent magnets for producing magnetic flux; 2, outer yokes; 3, a side yoke; 4, a center yoke; 5, gaps; and 6, a coil for providing a drive force when energized.
In the electromagnetic drive actuator thus constructed, the magnetic flux produced by the permanent magnet 1, as indicated by the broken lines in FIG. 2, passes through the air gap 5 while intersecting the coil 6 and through the center yoke 4, the side yoke 3 and the outer yoke 2 to return the permanent magnet 1, thus forming a closed magnetic path. Therefore, when current is applied to the coil 6, a drive force is provided in the direction of the arrow as indicated in FIG. 2. To make understanding of this invention clear, FIG. 2 shows only the magnetic flux formed on one side of the electromagnetic drive actuator.
The conventional electromagnetic drive actuator thus designed has a disadvantage that all the magnetic fluxes caused in the electromagnet 1 pass through the center yoke 4, the outer yokes 2 and the side yoke 3 to thereby make the magnetic flux density high, and therefore those yokes are required to be designed relatively large in cross section. This requirement is strong when the distance of movement of the coil 6 is increased.
In addition the conventional actuator has another disadvantage that it is unavoidably large in width, since the center yoke 4 is inserted into the coil 6.